To expand the storage capacity of your Mac, you can connect one or more external drives, including solid-state or traditional moving-platter mechanisms as well as flash-memory-based thumb drives. Regardless of the physical size, capacity and medium your external drive uses, it must meet simple but specific specifications to work with your Mac. If you choose a medium or enclosure that doesn't match your Mac's requirements, you may be unable to use it as a compatible choice.

Port Availabilities

An external hard drive hooks up to your Mac via a cable or built-in connector that connects from storage medium to computer. Drive cases may offer one or more of a range of ports, including USB, FireWire or Thunderbolt. If you buy an empty case and install your own hard drive in it, you can use port selection as part of your criteria for choosing casework. If you buy a drive/case combination, you must choose one that offers the ports you need for compatibility with your Mac. Depending on the age of your Mac hardware, it may include a USB port that supports only an older version of the USB specification -- which provides data throughput too slow for drive use -- or it may lack FireWire or Thunderbolt ports altogether.

Drive Format

PCs running Microsoft Windows use a drive formatting scheme called NTFS, which superseded FAT32 as the first choice on Windows systems, overcoming FAT32's 4GB limit on individual file sizes. Although Macs can read and write to drives formatted as FAT32, they can have trouble accomplishing the same objectives with NTFS drives and partitions. Macs running OS X 10.6 Snow Leopard or later can read from NTFS disks, but as of February 2014, no version of OS X can write to this disk format without a third-party utility.

Correcting Format Problems

If you select a drive in the Finder and press "Cmd-I" to open a Get Info window, you can see the name of the formatting scheme applied to it. To overcome the read-only obstacle posed by NTFS, use the Disk Utility application that ships with OS X to reformat your drive as Mac OS Extended (Journaled) for Mac-only use or FAT32 for storage-only compatibility with Macs and PCs. Note that the process of reformatting your drive erases any and all files it contains, so back up its contents first. FAT32 won't work as a bootable disk format on a Mac, and it makes a poor choice for iPhoto libraries.

Other Considerations

To install a copy of OS X on an external drive for use on an Intel Mac, the mechanism must be formatted as Mac OS Extended (Journaled), also known as HFS+, and the partition must use a GUID partition scheme. This differs from the requirements for use with a PowerPC system such as a G4 or G5, so if the drive you plan to use for external startup with a new Mac formerly served as a startup drive for an older system, you must reformat it before you can install OS X on it. If your external drive serves as a file archive and meets your Mac's format-compatibility needs, the mechanism may have developed problems that require the use of Apple's Disk Utility or a third-party drive-repair program before you can use the drive successfully.

About the Author

Elizabeth Mott has been a writer since 1983. Mott has extensive experience writing advertising copy for everything from kitchen appliances and financial services to education and tourism. She holds a Bachelor of Arts and Master of Arts in English from Indiana State University.